I DO NOT install that fitting. I realize that it's there to make the T/C lockup firmer, but without the fitting it will still lockup plenty firm.
The dangers of leaving that fitting in are:
1) The possibility of creating too much back pressure in the torque converter which will ultimately cause more push on the crank thrust surface, possibly causing crank thrust overload and failure. A very nasty contemplation.
2) Small particles building up to block the small orifice, thus blocking cooler flow which will then cause no lube oil for the transmission, no cooling of the transmission fluid and the possibility of excessive buildup of back pressure in the torque converter, which brings us back to the first item.
I have removed this fitting from customer's cars with debris blocking the hole as described in item 2.